The invention concerns a low-frequency alternating current signal detector, in particular for recognizing tone or possibly charging signals in a central office line interface circuit (hereinafter COLIC) connecting a private telephone installation to an analog telephone network subscriber line.
COLICs connect a private telephone installation, in particular a switch or a key service unit, to a local central office, which is usually part of a telephone network, to enable telephones or terminals connected to the private installation to communicate with telephones or terminals connected to the network but not connected to the installation.
At present, many network subscriber lines are still of the analog type and transmit speech signals in analog form on two wires in an audible frequency band between 300 and 3 400 Hz. The two line wires are also used to transmit signalling, for example in the form of alternating current signals at specific frequencies, either pulsed or of relatively long duration.
Existing private telephone installations usually employ time-division switching with the result that the speech signals are coded in digital form. This type of switching is well suited to the transmission of speech and data signals on the same media according to the changing requirements of users. It is therefore necessary to detect alternating current signalling in order to convert it into digital form on entering the installation in which it is to be processed.